1. Field of the Invention
The present invention relates to waste heat recovery from power transformers and more particularly to a process and apparatus for extracting useful energy from cooling oil in large power transformers.
2. Description of the Prior Art
Electrical generating plants require power transformers to produce the high voltages needed for power transmission. Losses occur in these transformers due to eddy currents, hysteresis, dielectric loss, I.sup.2 R loss from the exciting current, and the load losses due to the I.sup.2 R loss from the load currents. Although such transformers are quite efficient, for example in the range of 96-99%, significant amounts of heat are generated due to the extremely high powers handled by the transformers.
As an example, a transformer operating with 100 megawatt generators may have a no load loss of 100 kilowatts plus a load loss of about 170 kilowatts at 75 MVA load equivalent to 921,240 btu/hr of waste heat. Such a transformer may have a core and coils weighing about 170,000 pounds, an 83,000 pound tank, and a total volume of oil weighing about 110,000 pounds. It has been estimated that in a typical 50 MVA power transformer, the cost of the waste energy due to these losses will be over $62,000 a year at present day energy costs. Presently, practically all utilities utilize air coolers for removing the heat from the transformer oil thereby rejecting the heat to the ambient air.
In view of the large amounts of waste heat from power transformers, methods and apparatus for recovering this heat for useful purposes can significantly improve the operating efficiencies of power plants and power stations. Of the various possibilities, the most promising technique is the use of water cooling of the oil, raising the temperature of the cooling water to a sufficient level that the hot water may be put to other uses. Thus, one problem faced in this approach is to obtain a high enough water temperature to be useful. In a system utilizing flow of water through a heat exchanger to cool the oil, the output temperature of the water will be a function of the rate of flow. A low flow rate will produce a higher temperature of the water and also higher temperatures in the transformer. On the other hand, a high flow will transfer a greater amount of heat from the transformer to the water but the water temperature will be lower. Therefore, a balance between heat rejection and heat recovery must be achieved.
Fortunately, there are many uses for water having a temperature range of 140.degree.-160.degree. F. and therefore a system which will produce temperatures in this range will be useful. Another problem involves transferring of the heat from the oil to the water. Therefore, an efficient heat exchanger will be required. To effectively use the available hot water, it is necessary that the user be relatively close to the source of the heated water. The use of the hot water produced may, in many installations, be used by equipment at the power plant. In other cases, the utility could sell the hot water generated to customers living nearby at a lower cost than possible with the use of electricity or gas for water heating. This latter application is attractive since it would help reduce peak load requirements by reducing the customers' electric energy usage for water heating.
Water cooling has been used for transformers in the prior art but was usually implemented by disposing copper coils inside the transformer tank just below the surface of the oil. Water was then circulated through the coils to cool the transformer oil. However, water fouling and leakage produced problems with these systems and the air cooling systems are much more common at the present. When transformer manufacturers are requested to supply water cooled systems, it is common for them to supply a simple shell and tube type heat exchanger to replace the air cooled radiators in a typical transformer. However, this type of heat exchanger has a primary objective of heat rejection and does not provide means for achieving a balance between heat rejection and heat recovery.